1. Field of the Invention
The present invention relates to a digital signal exchange equipment for connecting a plurality of digital signal input circuit lines to an arbitory circuit line of a plurality of output circuit lines and to a circuit line selection control apparatus for selectively controlling a circuit line connection and for preventing a connection error at an exchange time.
2. Description of the Related Art
In general, a digital signal exchange equipment for performing an exchange of a fast and continuous digital signal, such as an image PCM signal used, for example, at a broadcasting station comprises a matrix array of switching elements each provided at a crosspoint of each input line with a corresponding output line, or at a crosspoint of each output line with a corresponding input line, with the input and output lines connected to input and output circuit lines, respectively, as a lattice-like structure. In this matrix array, any input circuit line can be connected to any output circuit line by making a selective, switching connection to a predetermined switching element.
A matrix type unit for controlling a crosspoint connection by the switching element requires a very large number of switching elements with an increase in the circuit lines and, furthermore, the switching of the switching elements is very complex to control. Since, in this case, an input digital signal has to be passed through a great many of switching elements until it is output from the exchange equipment, it is not possible to, due to a high-speed requirement for a digital signal to be transferred, disregard a delay of that signal when it passes through the switching elements. As the switching elements, use may be made of electronic switches (semiconductor switches) in which case the signal passage characteristics of this type of switches are generally not equal in terms of the delay of a signal at a time of a rise and a fall. For this reason, the input digital signal suffers a deformation with respect to time, each time it passes through the switching element, so that it has a very great deformation. This causes a difficulty in a signal identification.
As a general countermeasure against this situation, a method may be adopted which achieves the synchronization and waveform-shaping of such digital signal by a flip-flop each time that signal passes through the switching element at the crosspoint. If, however, this method is applied to an exchange equipment having a simple matrix array as set forth above, a great number of flip-flops are required in the case where many circuit lines are involved. This system dissipates more electric power and is difficult to implement because of many flip-flops involved, failing to offer any practical countermeasure.
On the other hand, a circuit line selection control apparatus used in the conventional exchange equipment performs a selective switching among the many circuit lines in a time-division fashion and dynamically monitors the switching elements so that a connection error may be prevented. In the time-division exchange system, however, the select signal is multiplexed in a time-division fashion to conform to the characteristic of the digital signal transmitted and then input to the exchange equipment. In the treatment of a fast, continuous digital signal, such as an image PCM signal, the bit rate of the select signal after being multiplexed becomes an extremely high level. It is, therefore, not easy to implement such a system. The error connection preventing means works under dynamical control of the switching element, only after a switch connection has been achieved, and it is not preferable that it be employed for such a purpose.
As set forth above, the conventional digital signal exchange equipment involves a high-speed requirement for transmitted digital signals as well as an increase in a signal delay among the circuit lines resulting from a large increase in the number of circuit lines. It is thus difficult to meet the high-speed requirement for digital signals as well as an increase in the number of circuit lines. Since the conventional circuit selection switching apparatus performs a selective switching among circuit lines, it is not easy to achieve it as a practical apparatus due to an extremely high bit rate of the select signal. Indeed in spite of the dynamic monitoring of the switches, the apparatus works only after a switch connection is achieved, failing to be manufactured as a fully practical apparatus.